MIMO multi-access passive time reversal communications

A recent paper [Song et al., IEEE J. Oceanic Eng. 31, 170-178 (2006)] demonstrated multiple-input/multiple-output (MIMO) multi-user communications in shallow water using active time reversal where the time reversal array (i.e., base station) sent different messages to multiple users simultaneously over a common bandwidth channel. Passive time reversal essentially is equivalent to active time reversal with the communications link being in the opposite direction. This paper describes MIMO passive time reversal communications which enables multiple users to send information simultaneously to the time reversal array. Experimental results at 3.5 kHz with a 1-kHz bandwidth demonstrate that as many as six users can transmit information over a 4 km range in 120-m deep shallow water using QPSK modulation, achieving an aggregate data rate of 6 kbits/s